A variety of methods have been proposed for cooling or chilling a substance or substances present in a polymerization vessel. For instance, a polymerization vessel to be used for production of polyvinyl chloride may be provided with a reflux condenser or may be cooled by means of cold water from a chiller. However, in the polymerization process where the reflux condenser is provided for the polymerization vessel, there is a tendency for a vigorous foaming of the contents to occur in the reaction system when cooled by the condenser. In particular, this phenomenon during the initial stage of polymerization results in deposition of bubbles of the initiator-containing dispersion liquor onto the inner surfaces of the gas phase section of the polymerization vessel and of the conduits connected thereto and also onto the inner surfaces of the reflux condenser and eventual formation of undesirable scales there. The thus formed scales may disadvantageously reduce the heat removal efficiency of the reflux condenser and also may present problems of poor quality of the product. For example, generation of "fish eyes" due to detachment of the scales and contamination of the product with debris of the detached scales. A great deal of labor is required for cleaning the polymerization vessel and incidental equipment by removing the deposited scales.
On the other hand, in the polymerization process where cooling of the polymerization vessel is effected by means of chiller water, the operating temperature is restricted within narrow limits so that this cooling technique may be acceptably employed only in the summer but is little effective in the winter. That is, the cooling technique is disadvantageous with respect to the thermal efficiency of the refrigerator used and does not provide a substantial enhancement in the cooling capacity. Further, in the case where the size of the polymerization vessel becomes large, it is well known that the coolng performance is relatively lowered, i.e., the conduction area available for cooling becomes insufficient relative to the heat generated by the polymerization reaction occurring in the large-sized vessel. Therefore, where a substantial magnitude of cooling is desired, a corresponding increase in the heat conductive area is required and hence the structure of the polymerization vessel becomes complicated and the equipment becomes expensive.